Present-day electrohydraulic steam-turbine control systems are generally equipped with electromagnetic electromechanical actuators that drive the pilot valves of hydraulic control-valve actuators. However, electromagnetic actuators have certain inherent disadvantages.
Their moving force usually depends on control signal values (amplitude). PA1 They cannot function in the absence of control signals. PA1 They trigger unintentional control-valve closures at a momentary dip in voltage.
Electromotor electromechanical actuators, on the other hand, are seldom used in the control systems for steam turbines, but they are totally capable of providing a full moving force independent of control signal values; in addition, they remain in their last position prior to either a momentary or a total loss of electrical power. Unfortunately, a drawback of electromotor actuators is that by themselves they cannot effect a trip action to shut down the turbine on demand during a complete electrical service interruption, unlike electromagnetic actuators that are, by nature, fail-safe.